1. Field of the Invention
The present invention is broadly concerned with an improved multiple-pass dryer useful for drying a variety of particulates such as wood furnish and agricultural products. More particularly, it is concerned with such a dryer apparatus which achieves substantially increased efficiency through use of an internal flow path arrangement serving to direct incoming, initially wet product along a serpentine flow path beginning in an outermost, relatively large cross-sectional area passageway and proceeding to successively smaller cross-sectional area passageways until dried product is removed from the apparatus. In this fashion the velocity of air currents within the drying apparatus increases as the currents pass through the dryer, whereby the velocity differential between such air currents and the saltation velocities of the particles being dried is maintained for maximum drying effect, and the net velocity of the particles moving through the dryer increases. In other aspects of the invention, apparatus is provided for the introduction of relatively dry ambient air into the innermost dryer drum to reduce the partial pressure of water vapor of the air currents passing through the dryer to increase final stage drying in the apparatus. Binders or other treating agents may also be added to the products during the final stage of drying through use of a novel addition conduit extending into the central drum of the dryer.
2. Description of the Prior Art
The drying of wood or agricultural particulates in a multiple-stage dryer is dependent upon a large number of factors, e.g., the type of product to be dried, the initial moisture content thereof, particle geometry, variable ambient conditions, dryer configuration and fuels being employed. Considerable research has been conducted in the past toward achieving maximum dryer efficiency, but in view of the relatively complex nature of the problem, the ideal dryer has yet to be developed.
In general however, the drying process involves several distinct phases or stages. That is to say, most hygroscopic materials exhibit several distinct drying rate periods as they pass through a multiple-pass dryer. Initial drying is accompanied by a warming of the material and its attendant moisture. The drying rate increases during this initial period, while the moisture content drops to a value which signals the beginning of a constant rate period of drying. During the constant rate period, moisture is evaporated from the surface of product particles at a steady rate until the surfaces are no longer entirely wet. Thereafter, a falling-off period obtains where the drying rate decreases because of the increasing difficulty of moving internal product moisture to the particle surfaces where it can be taken up and moved away. Finally, the product moisture is reduced to a point where an equilibrium is established with the surrounding atmosphere.
Conventional three-pass dryers include an elongated, horizontal, axially rotatable body having an outer drum and a series of concentric, smaller diameter drums within the outer drum. The respective drums are in communication with each other and define a serpentine flow path to the dryer. Without known exception, such dryers are provided with a product inlet oriented for directing initially wet product and hot drying air into the innermost, smallest diameter drum, whereupon the product is conveyed via induced draft currents through the outer drums until it reaches a passageway defined by the outer drum and the next adjacent inboard drum. At this point the product is in its final dried condition and is delivered for further handling or collection. Thus, conventional three-pass cylindrical dryers utilize comparatively high air velocities and temperature conditions in the innermost drum (first pass) where the incoming products are the heaviest and the wettest. Lower air velocities and lower temperatures obtain in the intermediate drum (second pass), and even lower velocities and temperatures exist in the outer drum (third pass).
This "inner drum to outer drum" configuration of conventional dryers is employed because it is believed that surface moisture evaporation is maximized in a relatively small cross-sectional area central drum where the highest air current velocity and temperature conditions exist. In the succeeding, larger diameter outer drums, it is believed that further drying is accomplished by phenomena characteristic of the falling drying rate phase. Also, the theory of conventional dryers is that the slower moving air currents in the outer drums allow larger particles to settle out and permit smaller particles to pass through, at least until the larger particles are dried enough to be picked up and conveyed by prevailing air currents.
In practice though, the relatively high air current velocity conditions in the first pass of a conventional dryer cause the wet particles to be quickly driven away from the heat source, and there is consequently a reduced opportunity for adequate heat transfer and evaporation. In subsequent passes with lower air current velocities, the particles may settle out because the prevailing air current velocities fall below the saltation velocity of the product (i.e., the minimum air current velocity needed to pick up and convey product at a given moisture level). Thus, plugging of the dryer may occur, particularly at high product flow rates, and at best the product only moves at a rate determined by the forward velocity of the slowest moving (largest) particles. The result is that the flow rate must be decreased and this inevitably has an adverse effect on drying efficiency.